County Cork comprises 1/10th of the land of Ireland and is the largest county. County Cork also had the second largest population in Ireland during the 19th century. Because of its size and large population the county was administratively divided into three dioceses: Cork, Ross and Cloyne.

When the Heritage Centres were set up, a centre was allocated for each diocese. Mallow Heritage Centre holds the records for the Diocese of Cloyne, which is comprised of 46 of the 120 parishes of Cork County.

The Heritage Centre for the Diocese of Cork was set up some years ago but will not be providing a research service for a number of years.

The Diocese of Ross incorporates most of West Cork. This area is known as a black spot in Irish Genealogy. A Heritage Centre was established for this area some years ago but has since closed due to legal difficulties - leaving Mallow Heritage Centre as the only centre that provides a complete research service in County Cork.